This invention relates generally to apparatus for automated chemical analysis and more particularly concerns the provision of a pneumatically controlled liquid transfer system particularly useful with such apparatus.
It will be appreciated that apparatus intended automatically to make a plurality of chemical tests upon a series of individual samples which are fed to said apparatus generally is known in the art. Such automated chemical analysis apparatus is utilized for example in the medical field, for diagnostic and routine informational purposes, and often for research. Chemical tests are performed on a series of individual samples. Often physical tests are performed additionally on these samples. Classically, all such tests were performed manually, by trained technicians in laboratories. In a typical chemical test in the medical field, for example, a sample of blood would be drawn from a patient, spun in a centrifuge to separate the plasma from the cells with the plasma decanted and placed in a container identified as to the patient. A small quantity of plasma would be measured into a reaction tube, mixed with a proper proportion of some chemical reagent, and maintained for a precise time at a precisely maintained temperature. Thus a certain chemical reaction would be permitted which will change the color of the mixture, providing a quantitative indication, say of the concentration in the plasma of the particular agent sought in the test being performed. The reaction tube would then be decanted into a cuvette, and a beam of light of predetermined wavelength directed through the cuvette, and, perhaps absorbance measured.
Over a period of years, these chemical tests have been developed to a relatively high degree of acceptance to ascertain such information as for example, the total protein of the blood, the presence of certain chemicals such as phosphorous, potassium, sodium and calcium; the amount of creatinine in the blood; the amounts of different enzymes, albumen, etc. Laboratories may perform as little as one or two tests on an available sample, or as many as twenty. The reagent composition, the proportions, the incubation time and temperature, etc. vary from test to test, but must be carefully monitored so that the results may comply with the requirements for precision and accuracy. Problems are inherent in the manual execution of these tests by a technician and hence, desired obviation of these problems have given rise to automated or at least semi-automated chemistry testing apparatus.
Among the problems associated with manual performance of the test concerned are the likelihood of human errors promoted by the measurements which must be made manually, the need for entering information and data relating to the sample and keeping its identification straight, tediousness and fatigue of the technician, errors in choosing the proper chemicals and using the proper proportions and the failure to keep the equipment clean of contamination. Loss of time, expense, waste, etc. are some other disadvantages encountered in performance of the classical analytical methods.
The art then has provided many different types of apparatus to perform automatic chemical analysis but resolving the problems inherent in varying degree with the classical analytical methods. Among structures utilized for this purpose, include those where turntables are employed which rotate to a sample withdrawing position. Samples are withdrawn thereat, diluted and passed to the processing portion of the apparatus. One form of apparatus involves the diluted samples passing through conduits one after the other separated by quantities of diluent and bubbles. In other systems, the diluted samples are carried in reaction tubes placed on continuous drums or conveyors.
One particular problem encountered in known automated chemical analysis apparatus involves the efficiency of transfer of liquids in such a manner as to provide quantitatively accurate dilutions and satisfactory physical transfer of the diluted test samples to the container within which they are incubated and later analyzed by way of optical or other methods. Transfer of such liquid materials is a serious problem in achieving precision and accuracy.
One persistant problem encountered involves the necessity to switch from one fluid source to another. Fluid valves, commonly known as check valves, are used to control the direction of fluid flow. Check valves are intended to allow fluid flow therethrough in one direction.
Generally, an apparatus utilizing check valves comprises a cylinder chamber with its associated cylinder head for changing the pressure within the chamber. The cylinder chamber has a pair of ports to which are coupled a pair of check valves having the direction of fluid flow respectively oppositely oriented. When the cylinder head is stroked to cause an increase in the cylinder chamber's volume, fluid is drawn into the chamber with the opening of the first valve and closing of the second.
The piston on the cylinder is stroked to decrease the cylinder chamber's volume whereby the fluid therein is discharged by way of the now opened second valve by way of the first valve being in closed position. Accurate volume measurements are required when quantities of liquid reactants can be transferred say from a source to a test tube or reaction tube for subsequent analysis. In some apparatus there are devices for drawing a precisely known quantity of first liquid, let us say sample by way of example, into a chamber, subsequently washing said chamber with a known quantity of diluent transferred thereto from a source thereof and thereby discharging the precisely diluted mixture to a delivery location.
The above described systems depend on check valves in the liquid conduits. These are quite often the least reliable elements of the system as a whole. Many check valves are subject to sealing or seating problems, which may be caused by sedimentary buildup formed upon the sealing mechanism and thereby introducing error due to the extra fluid that would leak through the valve when the proper sealing has not occurred. Other problems associated with the many prior valves involve the mechanical hysterisis or "backlash" that occurs due to the time delay between the time the pressure is changed to close the valve and the time that the valve actually closes. This "backlash" allows additional fluid to pass -- thereby to be dispensed, causing additional error in the volume measurement. It also has been found that "backlash" error is generally unpredictable because such error is a function of the back pressure applied to the valve to cause the valve to close. The back pressure is usually a variable in any pumping system and may be unpredictable.
Many prior art valves were subject to sealing or seating problems caused by swelling of the sealing parts of the valve so that the valve does not properly close or seal. Further, prior art valves of the character concerned are not interchangeably capable of handling corrosive fluids without utilizing special constructional material.
Other advantages will be evident in the course of the description of a preferred embodiment of the liquid transfer system and the particular valve therefor provided by the invention.